The present invention relates to a device for studying, in an accelerated way, the aging of a circulating fluid under specific imposed conditions.
Two different types of studies can be rapidly carried out with the device according to the invention. It can be used for assessing the particular qualities of a fluid formula, within the framework of operations of qualification of this fluid for a particular use or for determining the constitutive equations of a certain fluid in time, when it is subjected to working conditions reproducing the extreme conditions encountered in operation.
The device according to the invention can notably be used for studying and testing muds such as those utilized during operations for boring wells such as oil wellbores.
During oil boring operations, a mud circulation under pressure is established between the surface installation and the zone surrounding the drill bit. The mud is utilized for cooling and lubricating the drill bit, as well as for cleaning the borehole by conveying the cuttings up to the surface. It is also used for creating a hydrostatic pressure sufficient to stabilize the walls of the well and to contain the fluids under pressure in the formations surrounding the wellbore. While going through the drilled zones, the mud is therefore subjected to considerable stresses depending on the characteristics of the wells, whether there are very high temperatures up to 200.degree. C., high pressures of about several ten MPa or shearings at the level of the drill bit. The drilling conditions, the temperature and the nature of the formations frequently change. These changes may affect the properties of the drilling fluid. It is therefore important to make sure that the utilized fluid has the required properties and keeps them in time in spite of the stresses undergone.
The study of the behavior of drilling fluids can be carried out in the following manners.
Different measuring equipment can be included in the pumping plants delivering the fluid under pressure, in order to test, in a continuous manner, the characteristics of the fluid coming up from the drilling zone. Equipment for carrying out such tests is described, for example, in U.S. Pat. No. 4,635,735. The advantage of this type of measurings is that the studied fluid is subjected to the very conditions which prevail at the bottom of a wellbore. Nevertheless, its implementing requires the modifying and the adapting of an already existing installation for the circulation of a drilling fluid, which is not always possible.
The study of the behaviour of a fluid can also be performed in a laboratory. In an in situ installation, the measuring cycles are linked to the cycles of circulation of the drilling mud. The circulation rate being imposed and relatively slow, the study of the evolution of the fluid in time is generally very long. On the contrary, with laboratory equipment, the conditions prevailing in a drilling zone can be simulated concerning well-determined parameters by imposing a much faster circulation rate on the fluid. The fluid undergoes an accelerated aging thereby. Tests which would otherwise be very long are carried out rapidly. Besides, the possibility of varying at will certain experimental conditions reproducing the variations observed during the drillings allows multiple testings and measurings. Different equipment of this type are described in U.S. Pat. Nos. 4,483,189, 4,501,143 or 4,510,800.
The difficulty to be solved when a simulation equipment of this type is required consists in controlling the parameters acting on the studied fluid during each imposed working cycle well, by avoiding, at best, the uncontrolled parameter variations and, more generally, anything that might distort the measurements. Drilling fluids often contain corrosive (electrolytes) or abrasive (barium sulfate-clays) substances which, because of the testing conditions (temperature, circulation), are likely to attack the constituent materials of the circulation loops. The fluid can then be unintentionally contaminated by metallic ions which distort the measuring results. In the existing circulation loops, the heating of the fluid is most often performed in a single location. In order to take into account the limitations of certain equipment included in the loop, it is also compulsory to either cool the fluid or complicate the equipment. The experimental conditions achieved by the simulation loops do thereby not reflect at best the real conditions prevailing in the bottom of the wells.
It should also be noted that the equipment for studying circulating fluids become rapidly very complex if a complete set of measurings is desired. Maintaining during all the operations, on the fluid of the closed loop, precise experimental conditions such as temperature and pressure values become therefore very difficult to achieve.